Process for impregnating paper with ph controlled latex



Dec. 9, 1969 P. K. lsAAcs ET Al- PROCESS FOR IMPREGNATING PAPER WITH pH CONTROLLED LATEX Filed July 26, 1966 m. N M.

v' PHILIP K SAACS JAMES E.. GUTHRE INVENTORS BY @im 7 5 ATTORNEY United States Patent O U.S. Cl. 117-62 7 Claims ABSTRACT OF THE DISCLOSURE The invention disclosed is directed to a process for impregnating paper with a synthetic polymer latex having a pH value below about 2.4. The pH value of the latex may be adjusted with a mineral acid.

The present invention relates to a novel and useful process for preparing a product. More particularly, it relates to a process for impregnating paper.

It is known in the art that various latex impregnated papers may be used in the manufacture of pressure sensitive tapes, shoe insoles, artificial leather bases and the like. In the preparation of the latex for paper impregnation, two distinct systems are utilized. The first system utilizes a monomer, watersoap and an initiator which are mixed with heat and pressure. The second system utilizes the polymer, water, a quite large quantity of surfactant and energy which is generally imparted in the form of mixing to disperse the particles. The latex resulting from each of these systems, when applied to a paper, usually results in a bridging of polymer strings between the paper fibers and some actual penetration of the polymer into the fibers. The bridging results in a waste of polymer since the bridges add little, if any, to the strength of the paper. The penetration of the polymer into the fibers also results in a loss of polymer since it actually detracts from the strength of the fibers it has penetrated. In a copending application, one of the inventors of the present invention has discovered that an improved impregnated paper results i-f the latex is deionized prior to impregnation. The only drawback to this process is that in order to deionize the latex ion exchange resin must be used which requires regeneration periodically. Thus, the improved process adds costs over the old method. Quite obviously, if a process could be developed which added very little to the cost, it would receive widespread acceptance in the art.

It is an object of the present invention to provide a process for preparing an improved impregnated paper. A further object is to provide an inexpensive process for impregnating cellulosic pulp paper. A still further object is to provide a uniformly impregnated cellulosic pulp paper. Other objects will become apparent as the description of the invention proceeds.

These objects are accomplished by the present invention which provides a process for impregnating paper which comprises adjusting the pH of a synthetic polymer latex to a value below about 2.4 with a mineral acid and thereafter applying the latex to a cellulosic pulp paper.

In a preferred embodiment of the present invention the latex contains from about 5% to about 70% synthetic polymer and more preferably from about 15% to about 50%.

In the preferred embodiment, the pH is adjusted to a value of from about 1.7 to about 2.4 with a mineral -acid selected from the group consisting of sulfuric acid and nitric acid.

In a still more prefered embodiment, the pH is adjusted to a value of from about 2.0 to about 2.4 and 3,483,014 Patented Dec. 9, 1969 ICC the impregnated cellulosic pulp paper is at least partially dried to allow the latex to gel and then the acid is neutralized, preferably with sodium bicarbonate, before the final drying operation.

The term latex is used in its conventional sense to mean a dispersion of solid synthetic polymer particles in water which is capable of forming a film from the dispersion. The synthetic polymers which can be utilized in the latex include, without limitation, polymers and copolymers of butadiene, isoprene, chloroprene, acrylates, vinyl halides, vinyl esters and the like. Specific examples include the homopolymers polyethyl acrylate, plasticized polyvinyl chloride and the copolymers styrene-butadiene (40%-60%) and styrene-acrylonitrile-butadiene-acrylic acid (35%-14%50%-1%). Other polymers are likewise suitable.

The term mineral acid is used to signify any inorganic acid. It includes sulfuric acid, nitric acid, phosphoric acid, sulfurous acid, nitrous acid and the like. Preferably, the acid is used in a dilute form since concentrated acid will sometimes cause coagulation. If a concentrated acid is to be used the latex should be thoroughly agitated to prevent coagulation. The pH is generally adjusted using an automatic pH meter although indicators may be used or the required quantity of'acid calculated beforehand. The two preferred acids are nitric acid and sulfuric acid in that order.

The terminology cellulosic pulp paper is used to mean any conventional paper such as that made from wood pulp, cotton linters and the like. It also includes paper which contains some synthetic filaments as reinforcement.

The invention will now be described by reference to the drawings. In the drawings:

FIGURE 1 is an artists conception of an enlarged cross sectional view of a sheet of impregnated paper produced in accordance with the prior art; and

FIGURE 2 is an artists conception of an enlarged cross sectional view of a sheet of impregnated paper produced in accordance with the present invention.

In FIGURE 1 the paper fibers 1 have been penetrated by the polymer Z and strings of the polymer 3 bridge the individual filaments. The figure demonstrates that the prior art procedures result in a waste of polymer lby the bridges being formed and by the polymer penetrating into the fibers. v

In FIGURE 2 the paper fibers 4 are substantially uniformly coated with the polymer 5. The gure demonstrates that very little polymer is wasted when the impregnated paper is made in accordance with the present invention since the polymer is quite uniformly coated onto the filaments.

The following examples are given to illustrate the invention and are not intended to limit it in any manner. All parts are given in parts `by Weight unless otherwise expressed.

In the following examples, the tests are carried out as follows.

The pH of the solutions are measured using an automatiic pH meter (Beckmann Zeromatic).

The percent total solids in the latex is determined by accurately weighing a sample of the latex, drying said sample to constant weight and reweighing. The total solids are then calculated from the following formula:

measured in accord with ASTM-D41Z64T. The copolymer latex is dried in film form (3 mils thick), cut to 1A =pereent total solids sample strips, placed in the jaws (2" apart) of a tensile strength tester (Instron, Model TT, available from Instron Engineering Inc., Quincy Mass.) and separated at a rate of 0.5" per minute and a strain rate of 50% per minute at 23 C. and 50% relative humidity.

with the latex solids with the substantial absence of any strings or bridges connecting the bers. By taking 1/2 of the paper sheet before drying, the 1latex on the exterior of the sheet caribe removed by washing with Water leaving only that latex which has penetrated into the fibers. A

The d elammaun reslstan 1S measured by pt eparmg 5 comparison of the weight of the washed dried sheet with test sPecu-nen? Whlch are cui to .a length of approxlmaly the weight of the original sheets, shows that substantially 41/2 inches in machine direction and a width of apno latex has penetrated into the bers. In contrast, when proximately 2 inches 1n the cross direction of the paper. t i i t d th h th d The test specimens are conditioned 48 hours at 23 C, l0 a .con ro ae,x 1s came mug e Same proce ure and 50% relative humidity A 5 inch long Strip of rug without adiusting the pH with the exchange resin, a great binding tape iS piad on each side of the paper specimen deal of bridging takes place between the fibers of the so that the tape extends ybeyond the ends of the papen paper. Also, by the removal of the latex solids from the The specimen s than piad on the .bottom platen of a sheet, it can be determined that about 15% of the latex press so that approximately `0.5 inch of the tape extends Solids has penetrated im0 the interior 0f th@ berS- beyond the end of the platen. The specimens are pressed for 30 seconds at 275 F. at a pressure of 5.12 pounds EXAMPLE 6 er s uare inch on the bindin ta The s ecimens are oolegl to room temperature. gTheppecimets are cut in The Procdure of Example 1 s repeated employmg for strips approximately i inch Wide The two ends of the the latex solids a polyethyl acrylate homopolymer. A conrug binding tape are pulled apart into two approximately U01 1S 3150 1U aS 111 Example 1 equal plies. The ends of the binding tape are put into The properties ofthe resulting products are given in the the jaws of the Instron tensile tester and, after the first table below:

Polymer Tensile Elonga- Delaminauptake, strength, tion, tion Example pH Acid used percent psi. percent resistance Control 6 None 40 2,054 12 1.90

.......... 2 Nitric- 40 2,400 i0 2.20

inch of separation, the average force required to separate When the sheets are examined under the microscope, the paper is recorded at a crosshead speed to l0 as in Examples 1 5, substantially the same structure is inches/min. The delamination resistance of the paper nited. Also when the sheets are tested by removing the is reported in pounds based on the average values of latex from the exterior of the paper, substantially the 3 specimens. same results are noted. When the control is examined, it EXAMPLES 1-5 35 is found that its structure and properties are essentially To 1000 grains of a latex containing 50% of solids that of the control utlhzed above' (a copolymer containing styrene, 14% acrylonitrile, EXAMPLE 7 butadiene and 1% acrylic acid) is mixed 5% nitric acid until the pH has dropped from an initial value of 5 The procedure of Example l is repeated employing for to a value of 2. The inal solids content is 35-40%. A 40 the latex solid a copolymer of 60% butadiene and 40% standard size sheet (8% inches by 11 inches) of cotton styrene. A control is also run as in Example 1. The proplinters paper (l2 mils thick) is impregnated with the erties of the resulting products are given in the table belatex by dipping the paper into the latex to give an imlow:

Polymer Tensile Elonga- Delaminauptake, strength, tion, tion Example pH Acid used percent p.s.i. percent resistance Control 5 None 40 850 12 1.80

2 Nitrie 40 1,100 15 1.00

pregnated paper having 40% polymer solids (dry weight). When the sheets are examined under the microscope as The sheet is air dried at room temperature. A control in Examples 1-5, substantially the same structure is noted. is also carried out using no acid whatsoever. Alsol when the sheets are tested by removing the latex In Examples 2-4 the same procedure is repeated emfrorn the exterior of the paper, substantially the same rcploying respectively 5% sulfuric acid, 5% hydrochloric 55 sults are noted. When the control is examined, it is found acid and 5% phosphoric acid. In Example 5, the same that its structure and properties are essentially that of the procedure as Example l is carried out with the exception control utilized above. that after drying the sheet at room temperature the sheet is Washed with 1% sodium bicarbonate and then redried EXAMPLES 7 AND 8 at room temperature. Example 5 represents a particularly 60 preferred embodiment of the present invention. The prop- The PfOCefUfe 0f Example 1 1S repeated mploymg for emes of the resulting products are given in the table the latex solids a copolymer of butadiene and 40% below; acrylonitrile in Example 8. A control is also run as in When the impregnated sheets of the present invention r Example 1- In Example 9, the procedure 0f Example 8 are examined under a high powered microscope, it can be 6') is carried out with the added wash with sodium bicarseen that the fibers have been quite uniformly coated bonate as described in Example 5.

Polymer Tensile Elonga- Dclamina uptake, strength, tion, tion Example pH Acid used percent p.s.i. percent resistance C0ntrol 5 None 40 941 12 0.81 2 40 1,250 is 1.59

2 Phosphoric 40 900 15 1.20

The properties of the resulting7 products are given in the table below:

When the sheets are examined under the microscope as in Examples 1-5, substantially the same structure is noted. Also when the sheets are tested by removing the latex from the exterior of the paper, substantially the same results are noted. When the control is examined, it is found that its structure and properties are essentially that of the control utilized above.

The use of sodium bicarbonate (or any other neutralizing agent) in the above examples destroys any residual acids which if left in causes degradation and loss of tear strength. Accordingly, the procedure employing a basic rinse to remove the acid is a preferred embodiment of the present invention. Also, the paper may `be washed with a considerable amount of water which substantially lessens the eifect of the residual acid.

While in the above examples unmodied latex systems are utilized, it is obvious that latex systems containing anti-oxidants, fillers, pigments, oils, thickeners and the like can be employed.

Many equivalent modifications will be apparent to those skilled in the art from a reading of the foregoing without a departure from the inventive concept.

What is claimed is:

1. A process for coating iibers of paper which comprises, adding a mineral acid to a synthetic polymer latex and adjusting the pH thereof to a value below about pH 2.4 without coagulation, applying the acid treated uncoagulated latex having a pH value below about pH 2.4 to a cellulosic pulp paper, partially drying the acid treated latex to form a gel coating on the fibers, neutralizing any acid remaining on the latex coated fibers of the paper, and thereafter drying said fiber coated paper ywhereby the interior of the fibers is substantially free of said synthetic polymer.

2. The process of claim 1 wherein the mineral acid is sulfuric acid.

3, The process of claim 1 wherein the mineral acid is nitric acid.

4. The process of claim 1 wherein the acid is neutralized with aqueous NaHCO3.

5. A process for coating iibers of paper which comprises, adding a mineral acid selected from the group consisting of sulfuric acid and nitric acid to a synthetic polymer latex and adjusting the pH thereof to a value of from about pH 1.7 to about pH 2.4 without coagulation, applying the acid treated uncoagulated latex having said pH value to a cellulosic pulp paper, partially drying the acid treated latex to form a gel coating on the fibers, neutralizing any acid remaining on the latex coated fibers of the paper, and thereafter drying said ber coated paper whereby the interior of the fibers is substantially free of said synthetic polymer.

6. The process of claim '5 wherein the acid is neutralized with aqueous NaHCO3.

7. The process of claim 5 wherein the pH value is from about pH 2.0 to about pH 2.4.

References Cited UNITED STATES PATENTS 3,262,811 7/1966 Sellet 117-155 X 2,406,453 8/1946 Charlton et al. 117--155 X 2,416,272 2/1947 Wallach 117-62.l 2,422,573 6/ 1947 Lilenfeld 117-62.1 2,919,205 12/1959 Hart l17-62.2 X 2,950,214 8/ 1960 Smith 117-62.2 3,078,181 2/1963 Hart 117-62.2 3,282,724 11/1966 Atwell 117-62.2

FOREIGN PATENTS 69,470 9/ 1964 Canada.

WILLIAM D. MARTIN, Primary Examiner M. R. LUSIGNAN, Assistant Examiner U.S. Cl. X.R. 117-155 

